<!DOCTYPE html>
<html>
	<body>
		<style>
:root {
	--foo: if(x > 5) this.width = 10;
}
/* The rest of the CSS file */
#foo h1 {
	test: var(--foo)
}
		</style>
		<div id="foo">
			<h1 data-ruleset="test: if(x > 5) this.width = 10;">Header</h1>
		</div>
	</body>
</html>
